LOS ANGELES  As many as 1.5 million people are sickened by bacterial pollution on Southern California beaches each year, resulting in millions of dollars in public health care costs, a new study has found.

The study prepared by researchers at the University of California-Los Angeles and Stanford University is believed to be the first to examine illnesses at a large swath of the nation's most popular beaches. Previous studies have linked health problems to contamination at individual beaches.

"This helps us understand (the) risks and identify beaches where cleanup can yield the most benefit," said Linwood Pendleton, an environmental economist at UCLA and an author of the study.

The study, posted Monday on the Web site of the journal Environmental Science and Technology, covers 100 miles of shoreline in Los Angeles and Orange counties, which is visited by an estimated 80 million people annually.

The study found that between 627,800 and 1,479,200 "excess" cases of gastrointestinal illness occur at the beaches each year. That is beyond the number that would normally be expected.

Gastrointestinal illness is most commonly associated with swimming in contaminated water and causes such symptoms as stomach cramps, diarrhea and vomiting. The study did not examine the prevalence of other illnesses associated with polluted water, including eye, ear and nose infections.

Healthcare costs for illnesses related to beach bacteria range from an estimated $21 million to $414 million annually, depending on the method of reporting used, researchers found. Those estimates include direct losses, including missed work, medical treatment costs and doctor visits.

The study focused on 28 beaches during 2000. Researchers used bacteria measurements from surf, considered beach attendance estimates and extrapolated the health effects using two computer models -- one favored by the U.S. Environmental Protection Agency and the other by the World Health Organization.

Pendleton said the wide range in health and costs estimates existed because one method relies more on precise controls to account for illnesses and environmental conditions and less on self-reporting.

The study is expected to be published Aug. 15.

Among other findings:

-- Beaches at Doheny, Malibu, Marina del Rey, Cabrillo and Las Tunas had the worst water quality, while Newport, Hermosa, Abalone Cove, Manhattan, Torrance and Bolsa Chica had the best.

-- The three beaches with the lowest incidence of gastrointestinal illness were San Clemente's city beach, Nichols Canyon and Las Tunas, largely due to a smaller number of visitors.

-- Cleaning up storm water runoff, the chief cause of dirty ocean water in Southern California, would prevent 394,000 to 804,000 gastrointestinal cases and save $13 million to $28 million in annual health costs in Los Angeles County.

The state has spent an estimated $51 million on 66 projects in the past six years under its Clean Beaches Initiative, said Bill Rukeyser, spokesman for the state Water Resources Control Board.